1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image for developing an electrostatic charge image formed on the surface of a photoconductor in electrophotography, electrostatic recording or the like, a developer containing the toner, an image forming method using the toner, a toner container containing the toner, and an image forming apparatus equipped with the toner.
2. Description of the Related Art
In recent years, toners with smaller particle diameters have been actively developed at the strong request of the market for higher image quality, thus toners with an average particle diameter of 7 μm or less are currently on the market. The manufacture of above-described toners with a particle diameter of 7 μm or less requires much cost when using a conventional grinding method. To solve the problem, new pulverizing methods that replace the grinding method have been studied. Examples thereof include the preparation of toners by a suspension polymerization method.
It is a method suitable to obtain a toner that has desirable properties of the toner pulverized in such aqueous media, and has a small particle diameter.
However, toners pulverized in aqueous media have a very smooth surface, which is one of the properties of them. When toner particles have a small diameter and a very smooth surface, they are very difficult to be frictionally charged. Toner particles with a small particle diameter have very poor powder flow ability. In either of the one-component developing apparatus or the two-component developing apparatus, toner particles are frictionally charged while rolling on and contacting with the surface of either a developing roller or carrier particles, thus small-diameter toner particles that have poor powder flowability and a rolling property are hard to be frictionally charged, and thus are regarded as inferior in uniformity. In addition, when the toner particles have a smooth surface, the frictional charging property thereof is further deteriorated.
Although the mechanism has not been accurately elucidated, it is considered that a slip phenomenon occurs between a toner and a frictional charging member, which prevents the toner from obtaining a sufficient quantity of frictional charge. More particularly, it is considered that the smooth surface of the toner inhibits the toner from obtaining appropriate resistance against a toner layer thickness controlling blade used in one-component developing apparatus, or against a carrier used in two-component developing apparatus, thus the toner cannot obtain a sufficient quantity of frictional charge. In addition, when the toner particles are nonuniform in their frictional charge quantity, the frictional charge quantity results in broader distribution. Thus, if a toner could not obtain a sufficient quantity of frictional charge and has a broad distribution of frictional charge quantity, it develops even on a non-image area on a photoconductor, causing scumming.
Conventionally, in electrophotographic apparatuses, electrostatic recording apparatuses or the like, electric or magnetic latent images have been developed by toners. For example in electrophotography, an electrostatic charge image (latent image) is formed on a photoconductor, and the latent image is developed using the toner to thereby form a toner image. The toner image is usually transferred on a transfer material such as paper, and then fixed by heating or other methods.
Toners used for electrostatic charge image developing are generally colored particles in which a binder resin is to contain a colorant, a charge control agent, and other additives. The manufacturing methods are broadly divided into a grinding method and a suspension polymerization method. In the grinding method, a colorant, a charge control agent, an offset preventing agent and other additives are fused and mixed, and homogeneously dispersed in a thermoplastic resin. The resulting composition is ground and classified to obtain a toner. The grinding method can manufacture a toner with rather excellent properties, but the selection of the materials of the toner is limited. For example, compositions obtained by fusing and mixing must be those which can be ground and classified with economically usable apparatuses. According to the requirement, the compositions obtained by fusing and mixing must be adequately fragile. Therefore, when the composition is actually ground to particles, a particle distribution of a broad range tends to be formed. To obtain a copy image with a good resolution and gradation, for example, fine powder with a particle diameter of 5 μm or less and coarse powder with a particle diameter of 20 μm or more must be removed by classification, which significantly decreases the yield of the toner. In addition, under the grinding method, it is difficult to homogeneously disperse the colorant, the charge control agent or the like in a thermoplastic resin. Uneven dispersion of the compounding agents adversely affects the properties of the toner such as flow ability, development property, durability and image quality.
In recent years, to solve these problems in the grinding method, the manufacture of toners by the suspension polymerization method has been suggested and in practice. A technique to manufacture a toner for developing an electrostatic latent image by a polymerization method is known, and actually toners have been manufactured for example by the suspension polymerization method. However, toner particles prepared by the suspension polymerization method are spherical, and thus are inferior in cleanability. In the development and transfer of an object with a low ratio of image area, less residual toner is left and cleaning failure will cause no problem, but on an object with a high ratio of image area such as a photo image, the toner which formed an image that has not been transferred by a certain cause such as paper feeding failure may also occur as transfer residual toner, and accumulation thereof will cause scumming. The residual toner also contaminates a charging roller for contact charging a photoconductor, and inhibit it to deliver its intrinsic charging effect.
Thus, a method for producing toner of indefinite form by associating resin fine particles prepared by an emulsion polymerization method is disclosed (Japanese Patent (JP-B) No. 2537503). However, the toner particles prepared by the emulsion polymerization method have an abundance of residual surfactants not only on the surface but also in the inside of the particle, even after a washing process. This impairs the environmental stability of the toner charge, and broadens the charge distribution to cause a bad scumming on the resulting image. The residual surfactant also contaminates a photoconductor, a charging roller, a developing roller and the like, inhibiting them from delivering their intrinsic charging effect.
In the two-component developing apparatus, a toner is frictionally charged by contacting with a carrier, while in the one-component developing apparatus, the toner is frictionally charged by contacting with a supplying roller for supplying the toner to a developing sleeve, and by contacting with a layer thickness controlling blade for equalizing the toner layer on the developing sleeve. The charging property of the toner is important for the accurate reproduction of an electrostatic charge image on an image carrier such as a photoconductor, thus various kinds of charge control agents and methods to incorporate them into toners have been studied.
Charge control agents which function on the surface of toner particles, because of its high cost, have been attempted to be arranged on the surface of toner particles in a small amount. In Japanese Patent Application Laid-Open (abbreviated to JP-A, hereinafter) Nos. 63-104064, 05-119513, 09-127720 and 11-327199, charge control agents are attached to the surface of toner particles to impart the toner a charging property. However the charging property is insufficient and apts to be separated from the surface, and the manufacture method has not provided a desired charging property. In particular, the method is not intended to consider the initial charging rate of the toner.
JP-A No. 63-244056 describes a method for attaching a charge control agent to the surface of toner particles and fixing it on them using an impact strength occurring between a blade rotating at a high speed, which is referred to as a rotor, and projections fixed on the wall of a container, which are referred to as stator. An inner wall that is not smooth and has projections on it are likely to cause turbulence in a high-velocity airflow, thus it tends to cause excessive grinding of the particles, local fusion on the surface of the particles, embedding of the charge control agent below the surface of the particles, and uneven powder treatment. This seems to be due to the variation in the energy given between particles. More specifically, treatment through such a narrow gap may generate an abundance of heat due to an impact strength in an airflow, which causes the deformation of the toner particles and the progress of the grinding of the toner particles, resulting in the deviation of the average particle diameter and of the particle distribution from the desired ones. Besides, the charge control agent embedded below the surface of the particles might fail to fulfill its function. Regarding actual productivity, the quantity of the treated powder is extremely smaller in comparison with the space for treatment because of the heat generation and excessive grinding of the powder, thus the method is unsuitable to efficient production.
On the other hand, a fixing process by a contact heating method carried out using a heating member such as a heating roller requires the release property of toner particles from the heating member (hereinafter referred to as anti-offset property). The anti-offset property can be improved by arranging a release agent on the surface of toner particles. Regarding this, JP-A Nos. 2000-292973 and 2000-292978 disclose the methods for improving the anti-offset property not only by containing resin fine particles in the inside of toner particles, but also by unevenly distributing the resin fine particles on the surface of the toner particles. However, under these methods, the lower limit of fixing temperature increases, which causes the insufficient low-temperature fixing property or energy-saving fixing property.
However, the preparation of toner particles of indefinite form by associating the resin fine particles obtained by the emulsion polymerization method presents problems as described below.
When the fine particles of a release agent are associated with each other to improve the anti-offset property, the fine particles of the release agent are captured in the toner particles, resulting in the insufficient improvement in the anti-offset property. Since the toner particles are formed of randomly fused resin fine particles, release agent, colorant and other additives; the composition (the content ratio of the components), the molecular weight of the component resin and other properties vary among the obtained toner particles, which results in the difference in the surface properties among the toner particles, making it impossible to form a stable image for a long term. In a low-temperature fixing system that requires the low-temperature fixing property, fixing inhibition is caused by the resin fine particles unevenly distributed on the toner surface, this makes it impossible to secure the width of the fixing temperature.
The first object of the present invention is to provide a toner for developing an electrostatic image which has a small particle diameter essential for attaining a high image quality, is excellent in the frictional charging property, and can output a high quality image free from scumming.
The second object of the present invention is to provide a toner for developing an electrostatic image which can combine a high quality image and a low-temperature fixing property.
The third object of the present invention is to provide a toner for developing an electrostatic image which can provide a high quality image free from scumming, and good cleanability.
The fourth object of the present invention is to provide a toner for developing an electrostatic image which has a sharp charge quantity distribution, is excellent in environmental stability, and can form visible images with a good sharpness over the long term.
The fifth object of the present invention is to provide a developer containing the toner, an image forming method using the toner, a toner container containing the toner, and an image forming apparatus equipped with the toner.